Can I reduce voltage to electric cooling fan to reduce noise?e

I am planning to install an electric fan, problably either a Mark VIII or a Windstar, before spring. I have not heard the fans in operation but I have heard some electric fans that make a lot of noise. If the Mark VIII pulls 5000 CFM I would assume it makes quite a bit of noise. I will try it wired direct but if it seems to be too loud I thought I would try to rig it up so it runs at two speeds or variable speeds if that is reasonably feasible.

I assume I can do this by running two thermostatic switches, and setting one 10-15 degrees cooler that the other. I would then use the the T/S that comes on first to send reduced voltage to the fan. The reduced voltage should slow the fan down and reduce fan noise. If this does not provide enough cooling the engine will warm up enough to activate the second T/S, it would provide full voltage to the fan and maximum cooling.

Will reducing voltage to the fan motor damage it?

What can I use to reduce the voltage?

How much voltage reduction would you suggest?

Is there an easy way I can make the voltage adjustable?

How should I wire it?

Do I need to put a diode in each power supply wire? If so, do I need Heavy Duty Diodes, what size and where would I get them?

I think a two speed is all I will need but might be nice to have it variable speed. Any suggestions on how to do that?

Thanks, Joel